The present invention is a method for recovering particulate silicon essentially free of silicon containing compounds from a by-product stream containing metal salts and high-boiling silicon containing compounds having a boiling point higher than the sublimation temperature of the metal salts. The method comprises first separating particulate silicon and other solids from the by-product stream by a method such as filtration. The particulates are then washed with a solvent having a boiling point lower than the sublimation temperature of the metal salt, thereby separating residual high-boiling silicon containing compounds from the particulate silicon and other solids. The particulate silicon and other solids are then recovered in a dried form, essentially free of silicon containing compounds, by spray drying at a temperature lower than the sublimation temperature of the metal salt.
Commercial process for producing chlorosilanes typically use fluidized-bed reactors in which particulate silicon is reacted with, for example, methyl chloride to form organochlorosilanes or hydrogen chloride to form chlorosilanes. During conduct of the fluidized-bed process a portion of the particulate silicon, typically referred to as "fines" exits the reactor along with product and feed gases. These fines can comprise as much as ten weight percent of the particulate silicon fed to the reactor. Therefore, these fines can represent a substantial volume of material whose final deposition must be managed in an economical and environmentally sound manner.
Also during conduct of the direct process, metals, such as aluminum, which are present as contaminates in the feed materials may be converted to metal salts such as aluminum trichloride. The sublimation of such metal salts during subsequent separation processes can cause clogging of separation equipment such as distillation columns, spray dryers, condensers and the like.
The gaseous mixture exiting the fluidized-bed reactor is typically distilled at a temperature where the desired organochlorosilanes or chlorosilanes are recovered as overheads and the silicon fines, higher-boiling silicon compounds, and metal salts are recovered as a liquid stream from the bottom of the still.
The present invention provides a method where the silicon fines and other solids can be recovered as a dry powder essentially free of silicon containing compounds by spray drying. Burgie et al. teach a spray drying process for separating particulate silicon from a liquid by-product stream containing silanes at a temperature within a range of about 35.degree. C. to 750.degree. C. In their Example, Burgie et al., U.S. Pat. No. 5,118,486, describe the liquid by-product stream as one recovered from a chemical vapor deposition process for making high purity silicon. Since chemical vapor deposition processes for making high purity silicon employ a high purity silane as the deposition gas, the problem of the presence of metal salts which can sublime and clog the spray drying equipment is not present.
The objective of the present invention is to provide a method where a spray drying process such as taught by Burgie et al. can be used to recover silicon fines free of silicon containing compounds in the presence of metal salts. This objective is achieved by washing the particulate silicon with a solvent having a boiling point lower than the sublimation temperature of the metal salt, thereby removing the higher-boiling silicon containing compounds and any residual monosilanes. The solvent can then be separated from the particulate silicon by a spray drying method such as taught in Burgie et al. at a temperature below the sublimation temperature of the metal salt.
Particulate silicon recovered by the present method is essentially free of silicon containing compounds and may therefore be more easily recycled to processes such as the direct process or may be disposed of by more economical environmentally appropriate methods.